This invention relates in general to tapered threaded pipe joints. More particularly, but not by way of limitation, it relates to methods and apparatus for controlled joining or make-up of tapered, threaded pins and boxes.
Pipe sections in lengths of about twenty-six (26) feet to about forty-five (45) feet with outside diameters of about 23/8 inches to about 20 inches are commonly used in drilling, completion and production of oil and gas wells. The pipe, depending on the outside diameter, is referred to as casing or tubing and also broadly identified as OCTG (Oil Country Tubular Goods). The lengths of pipe with larger outside diameters, called casing, are run inside the well bore during the drilling process. The purpose of casing is to hold the walls of the well bore in place, separate formations, provide an interior space for completion and production operations and seal off formation pressures from the well bore. Tubing is installed inside the cased well bore for the purpose of bringing oil and gas to the surface from the well depths. These lengths of pipe usually have tapered, exteriorly-threaded male ends called "pin threads" or "pin ends". Such ends are threaded into couplings or collars (which are short female pipe sections, the threaded portions of which are often called "boxes"), there being an interiorly-threaded tapered box region at each end of each coupling or collar. Each thread has various thread elements which must be machined to close tolerances so that when the pipe and couplings are made up (coupled together or formed), one continuous pressure vessel is formed.
Tapered, threaded joints are very difficult to make up (form) properly. For example, because the male and female threaded regions are tapered (frustoconical), there can only be a certain amount of penetration of the pin end into the box of the coupling before permanent deformation of the threads occurs. Such permanent deformation is not desired for various reasons including the fact that the pin end and coupling are not reusable. In extreme cases the coupling may split. Conversely, however, there must be sufficient penetration to achieve good sealing against gas or oil leakage, to achieve adequate resistance to axial tensile stresses, etc.
The success of a connection or a made up joint, when the threads are machined to proper tolerances, is reliant on other factors to provide a leak-proof connection or make-up. There are basically three major factors or variables which determine whether or not suitable seal of mating threads can be obtained and the joint between two tapered pipe sections is correctly made. The first factor or variable is the American Petroleum Institute (API) Modified Thread Compound used to lubricate and provide solids to fill the voids within joining threads (see API Bulletin 5a2). The second factor or variable is application of the correct amount of torque to join the two threaded surfaces. Torque is the measurement of force in foot-pounds required to overcome resistance generated by interference between pin and box threads during make-up. Make-up torque is namely the torque which exists at the very last increment of the make-up procedure. Proper make-up torque for round thread casing and tubing is identified in API Bulletin 5C3-9.1, which is hereby incorporated by reference as though fully set forth herein, and calculated from formula 4.1.2 for joint strength in the same bulletin. Minimum, optimum and maximum torque values are listed in API Bulletin 5C1 which is also hereby incorporated by reference as though fully set forth herein. The third factor or variable is the amount of stand-off as measured linearly from the coupling field end (open end of the coupling) to the pin end of the pipe after proper make-up. Stand-off is related to the amount of penetration of the pin end into the coupling.
Several methods have been used to monitor and control make-up of oil field tubular connections. One approach is the "torque-only" method based on read-out of a load cell attached to the power tongs and calibrated for torque. Another approach is the "torque-turn" method based on a reference torque. For example, U.S. Pat. No. 3,745,820 discloses method and apparatus for controlling pipe make-up which uses a predetermined reference torque. A torque panel meter and three control knobs allow an operator to input the predetermined reference torque, the minimum torque required for make-up and the maximum allowable torque. Three "turns" setting controls allow an operator to input low number of t urns, intermediate number of turns and maximum number of turns. After this predetermined reference torque is reached during the rotation of the pipe, the torque is monitored together with the number of turns of the pipe to determine acceptability of the make-up.
U.S. Pat. No. 4,962,579 discloses a method of connecting threaded pipe sections in which a registry mark of a prescribed width is placed at a predetermined position on the exterior surface of a pin member. The pin member and a box member are continuously screwed together until the torque is within a predetermined make-up torque range. The location of the face of the box member is then observed to determine if the face of the box member is located within the width of the registry mark for an acceptably made up joint. If not, the joint is rejected.
Although it has long been known that both proper degree of penetration of the pipe section into the coupling and proper torque value at the proper degree of penetration must be achieved, present methods of make-up do not now achieve these factors jointly with any degree of regularity. The present invention provides apparatus and method for precisely controlling the make-up of tubular connections by combining applied torque and stand-off tolerances to achieve an ideal interference fit.